LECTURES. 



ON THE CONSTRUCTION OF BRIDGES. 



By FAIRMAN ROGERS, 



PROFESSOR OF CIVIL ENGINEERING IN THE UNIVERSITY OF PENNSYLVANIA. 



[Continued from Appendix to Report of 1860, p. 150 ] 



All the structiTres for spanning openings which we have thus far 

 considered, have been so arranged that portions of them are subjected 

 to a tensile strain ; the materials used in them are adapted to resist 

 such a strain, and also to receive such fastenings or connexions as will 

 enable them safely to transmit the forces applied. 



The building material, however, which is most universally at our 

 disposal, stone, although it is admirably fitted to bear a compres- 

 sive force, is not adapted to resist tensile strain, and in making use of 

 it in bridge-building we must apply it 

 in such form and such a manner as to 

 overcome this difficulty. 



If we have two pieces of stone, 

 neither long enough nor strong enough 

 to span an opening in the way of an 

 ordinary beam, we may arrange them 

 as shown in Figure 43, preventing them 

 from separating below by the tie rod A B. 



Should it be desirable that the space between the stones or tim- 

 bers be left without any encumbrance, the tie rod A B may be re- 

 placed by two abutments, so 

 firmly seated on the ground 

 that the thrust of the struc- 

 ture will not be sufficient to 

 push them apart or to over- 

 turn them, as in Figure 44, 

 where the reaction of the 

 ground takes the place of 

 the tension rod. 



Now, if it be necessary to 



Fig. a. 



